api [nfc]: Better name messageIds parameter on updating flags

These aren't message objects, but rather just the IDs of messages.

The name is `messages` in the API, and that'd be a pain to change.
But we can at least fix it within our code.

All the call sites already use appropriately "ID"-mentioning names.

Author: gnprice

src/api/messages/messagesFlags.js

@@ -10,8 +10,8 @@ export type ApiResponseMessagesFlags = {|
 
 export default (
   auth: Auth,
-  messages: $ReadOnlyArray<number>,
+  messageIds: $ReadOnlyArray<number>,
   op: string,
   flag: UserMessageFlag,
 ): Promise<ApiResponseMessagesFlags> =>
-  apiPost(auth, 'messages/flags', { messages: JSON.stringify(messages), flag, op });
+  apiPost(auth, 'messages/flags', { messages: JSON.stringify(messageIds), flag, op });